Holding devices used in conjunction with lathes for mounting a workpiece between the headstock and tailstock of the lathe, are typically called drive centers, spur centers, or prong centers. These devices are intended to serve both the function of accurately locating the workpiece and rotating the workpiece (i.e., driving it), such that the device maintains correct centering and continued rotation as the operator applies torque on the workpiece by means of cutting tools.
Typical drive centers have a tapered shank that is received by an identically tapered bore in the spindle of the headstock of the lathe. Drive centers have a center point and a number of spurs or prongs. The spurs embed and engage in the workpiece and are retained there by the force imparted to the workpiece from the tailstock at the opposite end of the workpiece.
The workpiece is brought into its correct location against the drive center by placing the center point into a previously made indentation in the workpiece. The tailstock center is similarly engaged in the opposite end of the workpiece. After placing the workpiece between the lathe centers, the tailstock center is advanced toward the headstock so that the workpiece is sufficiently forced against the spurs of the drive center for holding and driving the workpiece. This process is referred to as “mounting” the workpiece on the lathe.
There are a number of drawbacks associated with known drive centers. One of the encountered problems stems from the fact that the density of the material of different workpieces can vary greatly. Due to this, different lengths of center points are required to effectively position the different workpieces. It is generally desirable to use a long center point for working with materials of low density, e.g., soft wood, and a short center point for working with materials of high density, e.g., hardwood. If the material is very dense, it is difficult to force the center point far enough into the workpiece so that the spurs will sufficiently engage with the workpiece. In this case, spurs are unable to impart the required torque and tend to slip against the workpiece. Many turners mistakenly attempt to compensate for this drawback by over-tightening the tailstock so as to increase the amount of force between the headstock and the tailstock. However, it is to be appreciated that a spindle should be turned with a minimum amount of force from the tailstock. The minimum force is defined herein as that force which is just enough to keep the work from slipping. It is known that the increase of force, between the centers, creates an excessive load on the headstock bearings and revolving center which results in premature wear and possible failure of the lathe. Further, the workpiece tends to deflect or flex from the centerline when it is “squeezed” too tightly by the centers. This flexing, sometimes called “whip,” leads to vibration of the workpiece and causes the chisel to “chatter” which creates marks on the workpiece which are undesirable and difficult to repair.
When turning workpieces between centers, it is essential that the center point in the headstock spindle run true. If any of these components are eccentric, the workpiece will be eccentric about the rotational axis any time the workpiece is removed and replaced, or reversed end-for-end. Eccentricity may result from manufacturing tolerances of the center point, the shank of the drive center or the tapered bore of the spindle. The workpiece often becomes eccentric when it is removed from the lathe and then not accurately re-centered when it is again remounted on the lathe. To overcome eccentricity caused by manufacturing errors of the drive center and center point and the spindle, if the center point is rigidly coupled with the drive center, the drive center can be set into the spindle and the lathe turned on and with the spindle and drive center spinning, the center point can be trued-up so as to create a new axis. After this, the drive center must always be aligned in the spindle as it was when the new axis was created in the center point. In this way, the turner can achieve accuracy greater than that which was originally built into the lathe, but this remedy is only available if the center point is rotationally fixed in the body.
Truing-up the center point requires the center point to be rotationally fixed to the drive center, and other known drive centers in which the center point is not keyed, cannot be trued-up.
Although some of the known drive centers may have spring loaded center points and adjustable spurs, a major drawback of the known drive centers lies in the fact that the spurs are exposed, thus creating a safety hazard. It is possible for loose clothing, sleeves, neckties, jewelry or the hair of the turner, for example, to get caught or to become entangled in the spurs as the drive center rotates. Due to the high rate of rotation of the drive centers and the power imparted by the machine, when something becomes entangled with the spurs, this often results in serious injury to the turner. The turner could also inadvertently brush part of his/her body against the sharp spurs, whether the spindle is in motion or not in motion. Further, a collision between the tool rest or a chisel and the drive center can cause injury to the turner and damage to the spur center.